Air Screws on Caburetors for yamaha RD350

[Still biking]

Got a question on the purpose of the air screw adjustments on RD350s.  Do they compensate for seal leakage as the seals wear so that you have the correct air/fuel mixture?  Otherwise, what is the purpose of this adjustment?  thanks!

[m in sc]

the fuel screws control the amount of air through the pilot circuit.this allows for the fuel to mix with the air before being discharged into the carburetor, then the intake. ideally, you want to be between 1.25 and 1.75 turns out from bottom, this gives the right velocity in the circuit so the fuel disperses correctly.

if your bike, for example, only runs right at 3/4 turns out, your pilot jet is too small and you should probably go up one.  Inversely, if your bike only runs ok at 2 turns out, it would probably be good to go down one size.

it has -nothing- to do with compensating for seal wear.

[Still biking]

But lets say you had an incorrect air setting, would that allow excess air to enter and potentially cause a seizure from overheating like leaky seals would do?

[quocle603]

It fine tunes your air mixture for 1/8-1/4 throttle.

http://www.iwt.com.au/images/effects3.gif

[m in sc]

But lets say you had an incorrect air setting, would that allow excess air to enter and potentially cause a seizure from overheating like leaky seals would do?

if the pilot is too low in size or clogged solid, and the aircrews are maxxed out past 2 turns, yeah. ive seena bike seize, on looong engine braking, on premix, nip up from the pilots (throttle closed solid) but its extremely extremely rare. so, no. look elsewhere if you are trying to diagnose a seizure.  the likelyhood of that happening is almost zero.

what exactly is the issue? 

[quocle603]

But lets say you had an incorrect air setting, would that allow excess air to enter and potentially cause a seizure from overheating like leaky seals would do?

Sounds like you might an airleak somewhere. I would do a leak down test, you can get one or just make one. Spray the areas with soapy water, base gasket, head gasket, intake manifolds, seals, and etc.

[Still biking]

I did a leak down test on this bike about 3 years ago and it was fine.  But I have driven it about 8-9000 miles since then. 

One cylinder seized, haven't taken it apart yet.  But I am suspicious of the air screw setting as the cause. 

I had an ignition problem (points condenser) and first tried to adjust air screws for better performance.  I later found the ignition as the culprit for poor high rpm performance.  I did a complete ignition tuneup and it ran fine after.   I neglected to return air screws to proper setting and then a seizure happened. http://www.2strokeworld.net/forum/Smileys/akyhne/banghead.gif That is why I am suspicious of air screw settings. 

Impossible to do another leak down test, isn't it with engine seized?

[m in sc]

i seriously doubt the air-screws being misadjusted is the cause of a seizure unless one fell out completely. i mean, where were they at?

you have other issues.

id just replace the crank seals, its not hard to do, and refesh the topend. recheck everything, look for possible other air leaks..

yes, you can do a leakdown even if its seized.

[quocle603]

Can you tell us more details on your bike:

Carb settings?
Stock timing?
Premix or oil injection?

[Still biking]

its completely stock, exhaust, ignition, carbs etc.  runs on pre mix.

From what I gather from a tech site, the air mixture screw does effect the air fuel mixture on the RD350 up to 1/4 throttle turn.  And I was riding the bike on a lazy ride under 1/4 throttle for the most part when the failure occurred.

The day was not hot, about 70 F and I was climbing a slight continous grade when it seized.  But poking along. 

[SoCal250]

its completely stock, exhaust, ignition, carbs etc.  runs on pre mix.

From what I gather from a tech site, the air mixture screw does effect the air fuel mixture on the RD350 up to 1/4 throttle turn.  And I was riding the bike on a lazy ride under 1/4 throttle for the most part when the failure occurred.

The day was not hot, about 70 F and I was climbing a slight continous grade when it seized.  But poking along.

Did you increase jet size to compensate for pre-mix?
Did you block off the oil injection ports on the carb bodies?

[Still biking]

I did not change any of the jet sizes and the oil ports were blocked.  It would runaway if they weren't. 

[m in sc]

you need to increase the jet sizes to run on premix. if you were running 25 pilots, modern fuels alone you should be tuned a bit richer, and then adding oil in there, you should have been 27.5-30 pilots minimally.  also, they aren't meant to be plodded along at low rpm for long.  what side seized?

keep in mind, premix actually makes the bike run (fuel) lean, it needs to be compensated for. 

[Still biking]

I find it hard to blame the size of the jetting for this.  I have ridden this bike for the past 3 years  with the same jetting for over 12,000 miles with no events.  At times in summer heat. 

And after I tuned the ignition I ran it real hard  (up to the redline sometimes) , for about a 50 mile trip to check the performance and had no issues.  If seal failure was the cause I believe it would have seized then.  The only way I could see it was a seal failure was if something catastrophic and sudden happened.

I did more research on air screw settings. Curiously the bike had a slight stumble at start which it never did before.  According to what I read, that was characteristic of a lean air screw setting. 

The bike has over 23,000 miles and the last leak down test and compression test were in spec the last I checked.  I did a compression test last summer. 

When I rebuild this I will do a leak down test after completion to check seals again. 

But I don't think that replacing seals is a simple job, removing the engine and splitting the case isn't easy for me. 

[m in sc]

not blaming the jetting, im just saying, it need to be compensated for to do it correctly. id pull the top end and inspect, itsall useless speculation at this point, but i can say im pretty darn sure its not the air screws, my guess is its just worn out